Heat-induced mutations in bacteriophage T4 have been demonstrated to occur by at least two distinct mechanisms: (1) G.C base-pairs are converted to A.T base-pairs (transversion). The C of the base-pair is the target of the first mechanism. The G of the base-pair is the target of the second mechanism. Studies of the step(s) required for conversion of the heat-induced lesion (pre-mutational lesion) to the fully expressed mutation are continuing. The specificity of the transversion reaction (G.C to T.A versus G.C to C.G) is to be determined. T4 phage contain glucosylated hydroxymethylcytosine in their DNA. The influence of this modification on mechanism (1) will be explored.